Some electrical devices require voltages within a specified range bounded by a minimum supply voltage and a maximum supply voltage in order to operate in a predictable manner. That is, in order to have a defined behavior where outputs are deterministic and causally related to inputs, an electrical device generally requires a minimum supply voltage and may not function at voltages greater than the maximum supply voltage. At voltages below the minimum supply voltage or above the maximum supply voltage, the electrical device may function, function in a limited manner, function erratically or in a temperature-dependent manner, or not function at all. Typically, the exact electrical device behavior outside a specified range of supply voltages is not deterministic. Some variation in supply voltage requirements can exist between one instance of a given electrical device (e.g., an integrated circuit) and another instance of the same give electrical device. For example, variations in manufacturing process parameters of two different production lots of the same integrated circuit can result in one production lot having one minimum (or maximum) supply voltage requirement, and a second production lot having a lower minimum supply voltage requirement (or higher maximum supply voltage requirement). To account for such variation, a manufacturer can determine, by testing, minimum or maximum supply voltage requirements of several production lots of a particular device, then add a margin to the highest (or lowest) determined supply voltage requirement to arrive at a specified minimum (or maximum) operating voltage requirement. A specified minimum operating voltage requirement generally represents a conservative minimum voltage, above which devices from any production lot are likely (e.g., 95% likely) to function in a deterministic manner (within other specified parameters, such as, for example, current and temperature ranges).
To ensure that an electrical device in a particular system functions in a deterministic manner, hardware designers can include conventional reset circuits in the system that can be used to prevent portions of the system from functioning at all if supply voltages are below specified minimum values. Conventional reset circuits can be particularly important at two different times to a system that includes electrical devices: during initial power-up, when the supply voltage of a system “ramps up”; and during brownout conditions that can occur during operation of the system, during which the supply voltage temporarily dips below a specified minimum voltage. During power-up, a conventional reset circuit can hold a device to which it is connected in a reset state until a corresponding supply voltage reaches a minimum voltage (what is commonly referred to as a power-on reset (POR) circuit). During normal operation, a conventional reset circuit can assert a reset signal to the device to which is connected in order to prevent the device from operating in an unpredictable or nondeterministic manner (e.g., possibly generating or propagating erroneous signals in the system). Some systems include multiple devices or circuits that each operate at different nominal voltage levels, and that can have different specified minimum voltage levels. Such systems can include multiple reset circuits (e.g., one reset circuit for each voltage level).